1. Field of the Invention
The present invention relates to radio communication using multiple-receiving/transmitting antennae. More particularly, the present invention relates to a radio communication apparatus and method for allocating transmission power to maximize the channel capacity.
2. Description of the Related Art
Channel response information between a transmitter and a receiver should be known to the transmitter and the receiver to detect the optimum channel capacity in radio communication. However, in general, it is assumed that the receiver knows channel response information, while the transmitter does not. Thus, channel response information must be fed back to the transmitter from the receiver. Here, ‘channel response information,’ as used in radio communication using multiple antennae, is information regarding the channel response between each transmitting antenna and each receiving antenna. The channel response information increases in proportion to the number of transmitting/receiving antennae. Therefore, the channel capacity of a radio communication apparatus using multiple antennae is increased in proportion to the number of antennae used. However, in this case, the amount of information to be fed back to the transmitter is also increased in proportion to the number of antennae.
Conventionally, allocation of transmission power is performed in a radio communication apparatus by an equal power allocation method and a water-filling power allocation method.
In the equal power allocation method, transmission power is allocated equally to base-band signals of transmitting antennae when channel response information is not fed back from a receiver to a transmitter. With this method, the channel capacity Ceq of a conventional radio communication apparatus can be calculated by the following equation:                               C          eq                =                              log            2                    ⁢                      det            ⁡                          [                                                I                                      n                    R                    ′                                    ′                                +                                                                            P                      T                      ′                                                                                      n                        T                        ′                                            ⁢                                              σ                        N                        ′2                                                                              ⁢                                      H                    ′                                    ⁢                                      H                                          ′                      ⁢                                                                                           ⁢                      h                                                                                  ]                                                          (        1        )            wherein I′ denotes an identify matrix, n′T denotes the number of—transmitting antennae, n′R denotes the number of—receiving antennae, P′T denotes the total amount of power available by the transmitting antennae, H′ denotes a channel response matrix having a size of n′R×n′T, σ′2N denotes the dispersion of received Gaussian noise, det denotes a determinant, and H′h denotes the conjugate transpose matrix of H′.
The equal power allocation method is advantageous in that channel response information need not be fed back from a receiver to a transmitter. Disadvantageously, the equal power allocation method causes the channel capacity to be less than in a radio communication method that feeds back channel response information from a receiver to a transmitter.
For a case where all channel response information is estimated by a receiver and then is fed back to a transmitter from the receiver, using the water-filling power allocation method, the transmitter may allocate transmission power to base-band signals using the limited total power as the determinant for maximizing the channel capacity. In this method, a conventional radio communication apparatus having multi-input and multi-output is converted into a radio communication apparatus having several parallel elements, with each having single inputs and single outputs, by decoupling conversion for completely canceling interference between signals. In such a decoupling conversion, a V matrix in the transmitter and a Uh matrix in the receiver are used to diagonalize the channel response matrix H′ through single value decomposition, using the following equation:UDVhH′=UDVh  (2)
A conventional water-filling power allocation method for maximizing channel capacity may be derived with the channel capacity Cwat of a conventional radio communication apparatus, which is calculated by the following equation:                               C          wat                =                              ∑                          k              =              1                                      n              ′                                ⁢                                    log              2                        ⁡                          (                              1                +                                                                            P                      k                      ′                                        ⁢                                          λ                      k                      ′                                                                            σ                    N                    ′2                                                              )                                                          (        3        )            wherein n′ is min(n′T, n′R), P′k denotes a power allocation value of each sub-channel, and λ′k denotes the ktheigenvalue of H′hH′.
In the water-filling power allocation method, the power allocation value p′k of each sub-channel that maximizes the channel capacity Cwat is measured using a ‘Lagrange multiplier’ as follows:                               p          k          ′                =                  {                                                                                                                1                                              λ                        0                        ′                                                              -                                                                  σ                        N                        ′2                                                                    λ                        k                        ′                                                                              ,                                                                                                  if                    ⁢                                                                                   ⁢                                          λ                      k                      ′                                                        >                                                            λ                      0                      ′                                                              σ                      N                      ′2                                                                                                                                            0                  ,                                                            otherwise                                                                        (        4        )            wherein λ′0 denotes an invariable obtained from the limited total power P′T.
When the water-filling power allocation method is adopted, an n′Rn′T complex number, that is 2n′Rn′T real numbers must be fed back to a transmitter from a receiver in a conventional radio communication apparatus. Therefore, with the water-filling power allocation method, the channel capacity can be maximized in a conventional radio communication apparatus, but a large amount of information must be fed back to the transmitter from a receiver.